If there is one thing that comes back to haunt contractors, it is not knowing whether or not the wood is acclimated to their jobsite. The old idea of “leave it on-site for a week or two” still prevails among those not familiar with how and when wood acclimates. Standard [U.S.] milling will dry wood to a 6-9 percent moisture content range. That is usually good for most of the homes with a Relative Humidity (RH) between 30-50, and temperatures of 60-80˚ F according to the U.S. Forestry Department’s chart familiar to those that have attended NWFA classes (see below).

Warning though, no two jobsites are alike. You need to know the RH and temperature the owners plan to live in so you know where the wood needs to be for the most stable outcome of your project.

Use your moisture meter and test the wood (40 tests per 1,000 SF) upon receipt, delivery to jobsite, and continuously if the job lasts longer than a few days. Test your subfloor as well (20 tests per 1,000 SF). Test the RH and temperature. Mark your findings and determine where the sweet spot is. In future tests, note any unusual variations that can save you from inadvertent de-acclimation from other trades. It’s best to record and photograph your tests and save them in the client file. Should something happen down the road, you have the data an inspector needs. So many contractors are caught “deer in the headlights” when a failure gets inspected and cannot produce the data the inspector requests.

Acclimation is perfect if you are on target to the RH/temp numbers according to the U.S. Forestry chart. Consider that there are elements not in your control that may persuade you to install when the floor is not ready. Avoid falling into this trap at all times. It will not matter to the GC, the owner, or the judge if the floor failed when installed improperly acclimated. You are the expert. You will be at fault for knowing better but still installing. Besides, how hard will it be to recover financially, not to mention mending the damage to your reputation?

As far as moisture meters go, there is no valid excuse as to why you cannot afford the most accurate, expensive testing kits available. To not spend the money on your meter, but gamble time and again on a floor not failing is foolish at best. Beg, borrow, find a way to get the best meter into your hands.

Take into account your special circumstances. Here in South Florida, I deal with crawl space homes that need wood acclimated up to 13 percent MC while slab homes that are well insulated and sealed off need 8.5 percent MC. My good friend Jason Elquest has the opposite conditions in Scottsdale, Arizona – they are so dry his meters are lucky to read moisture in his acclimated wood.

Learn your area, learn your meter, and make sure you take care of this very important part of your work.

To learn more about the importance of acclimation and how it affects wood, check out the recently updated NWFA Moisture and Wood Technical Publication.

Best of Luck!

10 thoughts

  1. Hi Lenny,

    Please advise what moisture meters you suggest. If we were to use a cheap meter, how accurate would the 40 tests per 1000 sq.ft. be and would the data be almost meaningless?


    – Chip

    1. Please understand that I cannot recommend one meter over another, except to say that do your research on the devices, customer support (important!) and their versatility. Select your meter and then stick with it. You then need to use it frequently to understand what its telling you. Not all readings are what they say they are. Simple example is testing an existing stable floor system only to note the flooring and subflooring may read 3-4% different (even after species corrections) but they are in equilibrium to the environment. So when I read wood at 8% and subfloor at 12%, on my meter, I know they are actually equal. Strange but true, that’s what I get on my meter and similar issues occur with other meters.

    2. Follow up for you Chip,

      Cheaper meters wont have the variability of internal species corrections, on board data logging/averaging and not use a probe with insulated pins. You are better off buying the meters that have these features. Mind you, there is a place for surface meters as a summary inspection tool, a “wet or dry” indicator of sorts that can be used without damage to the product. To get the real data though, you need to probe the wood with insulated pins, at the depth as close to center of the board thickness and away from the ends. I recommend using the probe meters over surface meters whenever possible.

      No 2 meters will read the same number on the same board read simultaneously. I demonstrate that at the schools. Does it mean one meter is more accurate than the other? To be sure, each meter must be tested against a calibration block of known resistance. That is why your meter should never be compared on par to another’s meter. It’s possible different data points get generated from the same group being tested.

      SIDENOTE: I’ve seen meters misused by placing the pins between boards to hide the holes. This is incorrect and will not provide accurate data. Follow the manufacturers instructions on how to properly use your meter.

  2. Lenny, What are your thoughts about using other finished goods as a decent benchmark as to the woods comfort zone as well as all the other tests we do. In the “old” days we always did that on remodel. As an example, I receive my flooring materials and find the MC to be 8%. After measuring the cabinets, staircases and baseboards I find the MC or them is also 8%, other than making sure that my subfloor is at the right levels may I proceed with installation without acclimation. Since our manufacturers produce at 6-9% are there not many times that I can proceed without acclimation after I have assured myself the wood is in the correct range? Does acclimation, especially on solid plank, really do any good when just the ends of the cartons are opened. Or do you just end up with wood plnks that have acclimated ends, and if humidity would cause the wood to swell somewhat, would you not end up with boards to wide on the ends with the appearance of poor milling?

    1. Hi Neil! That’s a lot to cover.

      Getting the RH and Temp data is crucial for sure. Testing other species/materials can provide data points if you “know” your meter and what it’s telling you on other species/materials (see my comment about plywood/wood data disparity). Ideally, test the same species in the house that you have for flooring.
      “Proceeding without acclimation” is an incorrect statement and that is the deeper understanding needed about acclimation. If all your data points indicate your floor test and the environment are at the Equilibrium Moisture Content already, and you have measured data that indicates the floor is where it is supposed to be, ergo, no acclimation (changing its moisture content) needed, then the installation may proceed. The floor was ready and no additional changes to it are needed.

      For on site acclimation of materials, there are many variables at play. The most prominent and misunderstood is the access of surface area to environment. Wood in cartons with ends open cannot possibly acclimate to the same effect as sticker stacked individual boards due to maximizing the surface area exposed to the environment of the latter. Add to this the attempt to acclimate surface coated products and the acclimation will occur faster at the uncoated exposed surfaces versus the coated face. Its common for manufacturers to state something along the lines of “open the ends of the boxes… for 3-4 days for acclimation” and this is simply wrong. I commend their attempt at advising the installer about acclimation, but their instructions are just wrong. Data has to come from a moisture measuring device, not a clock. The materials have to be maximally exposed for the fastest and uniform acclimation rates to occur. It’s the physics of molecular exchange at a boundary – in laymens terms: evaporation or absorption. In reality, nothing is stagnant, there is always an exchange going on.

      We could really get deep into the science and physics of acclimation so I’ll pause here and hit your final question: can the ends swell on opened cartons appearing like mismilled wood?
      The simple answer is yes, this is a possibility. More than likely to be seen in areas of very high differentials on RH and Temp compared to the mill’s planned moisture content target. Storage of flooring in non-climate controlled warehouses for extended periods of time can lead to ends swelling compared to the center of packed pallets in areas like Florida, Mississippi, Louisiana.

      Mind you that this can apply to a lesser degree on engineered products. Despite the universal impression that it is more stable is actually forced compliance. The opposing grain directions will build up stresses restricted by the adhesive and opposing wood fibers in adjacent layers. Don’t be fooled, the stress is there if you move the materials from the manufactured moisture content into another. Higher stresses for higher differentials.

      We should have a panel discussion about this at the Expo. We would probably need over 3 hours and likely not hit all the points in depth. LOL… I did one 2 years ago on Water Popping or Not adn it ate up all of the 90 minutes and still could have gone on!

  3. H Lenny. Very good article. Thank you. My question is this. Why doesn’t anyone every talk about what to do when the subfloor moisture content is dryer than the wood being installed. Everyone in the world advises what to do when measuring subfloor content. Two % with plank and 4% with strip. But what does the flooring do when the subfloor is dryer than the wood flooring. Will it shrink the flooring? Does it matter. I know NWFA has never funded the science because I talked to the scientists who do those types of studies. The problem is if the subfloor is that dry it’s probably because of environmental influences that are never going to change and what to do about it especially when installing a less stable plank floor. Logic would indicate raising the humidity in the subfloor by adding a humidifier or maybe wetting the subfloor. Problem is either way you creating a artificial environment and how would you ever know when equilibrium is achieved. I have run across this many times in my 35 plus years and have never been satisfied with the answer’s I’m getting from other professionals. Thanks in advance Mike Imel Sr.

    1. Hi Mike,

      Your “subfloor is drier than hardwood floor material” would indicate that your hardwood floor materials have to be acclimated to THAT drier condition. Certainly any wood materials placed in that environment will eventually come to balance (Equilibrium Moisture Content) and wood movement would be expected. In this situation, shrinkage is the result. Attempting to overcome permanent or constant dry environmental conditions mechanically with a humidifier may not be the most viable answer.

      Moisture meters,data recording information and possibly data recorders imbedded in the floor are your best defense should you be called for floor movement post installation.

      1. Thanks Lenny for replying. Yes I agree letting the flooring sit in the environment until it matches the subfloor is the one guaranteed method but something tells me it would take a very very long time. A good lesson for taking moisture readings prior to taking the job. I’ve had nwfa tech guys tell me there is no science to back up the fact that the flooring will shrink. Thanks

        1. The rate of moisture change has several controlable variables: air flow, surface area exposed, moisture gradient, construction. Any and all of these are contributors or detractors to acclimation.
          A pack of wood, not separated, even in the most arid or humid conditions, will acclimate only the exposed 6 surfaces (5 if its sitting flat to the floor) and a bit in to the ends of the pack (due to cell structure). Engineered cores also have difficulty acclimating in the core because there are glue layers delaying or even preventing moisture transfer within and under the lamela. More often, the lamela will change at a different rate to the core, leading to lamela failures from stresses imposed.

  4. So what is a installer to do when the subfloor is under 3 % and ambient is 40 % and the flooring is 6 to 8 %. It seems obvious that the subfloor is going to take a very long time to to come up to where it needs to be. Maybe pass on the job?

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